Conventionally, acrylic cross-linked resin particles and styrenic cross-linked resin particles have been typically used as a light diffusing agent contained in light diffusible members such as a lighting cover and a light diffusion film and a light diffusion plate for a liquid crystal display; a flatting agent for coating material; and other agents. In order to impart functions such as a light diffusion function and a flatting function to these resin particles that are to be mixed in a light diffusion film or to be used as a flatting agent for coating material, the resin particles are dispersed in a binder resin solution to prepare a dispersion liquid, and the dispersion liquid is applied onto a substrate to form unevenness on the surface (the surface of a light diffusion film or the surface of a coating film). In this case, the unevenness on the surface exerts the light diffusion function or the matting function. When mixed in a light diffusible member such as a lighting cover and a light diffusion plate (a light diffusion layer), these resin particles are kneaded into a transparent base resin such as polycarbonate and polystyrene to give a mixture, and the mixture is subjected to extrusion molding, injection molding, or other techniques, thereby forming a molded article. The molded article exerts the light diffusion function due to the surface unevenness of the light diffusion member or to the difference in refractive index between the transparent base and the resin particles and serves as the light diffusion member.
The resin particles having excellent properties such as light diffusion properties and flatting properties are required to have a comparatively small particle diameter. However, the resin particles having a small particle diameter, which readily fly as dust and have poor flowability, are difficult to be handled. To address the problem, a plurality of resin particles are aggregated by, for example, spray drying to yield a resin particle aggregate, which is easily handled.
For example, Patent Literature 1 describes a particle aggregate including cross-linked polymer particles that are obtained by polymerizing a vinyl monomer mixture containing divinylbenzene or trimethylolpropane tri(meth)acrylate and a non-cross-linkable monomer.
Patent Literature 2 describes a powder composition that can be redispersed in water and is obtained by spray-drying a copolymer dispersion mixture including vinyl chloride, ethylene, and a copolymerizable functional monomer such as acrylamide in the presence of an anti-antagonist such as an inorganic extender pigment.
Patent Literature 3 describes a polymer particle aggregate obtained by spray-drying a slurry containing polymer particles that are obtained from a monomer mixture containing a monofunctional monomer and a polyfunctional monomer, a surfactant, an inorganic powder, and an aqueous medium.